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Supernovae High Energy Astrophysics [email protected] http://www.mssl.ucl.ac.uk/ Introduction • Supernovae occur at the end of the evolutionary history of stars. • Star must be at least 2 M; core at least 1.4 M. • Stellar core collapses under force of its own gravitation. • Energy set free by collapse expels most of star’s mass. • Dense remnant, often a neutron star, remains. Nuclear binding • M nuc(A, Z) < ZMp + (A - Z)Mn • M (A, Z) = ZM p + (A - Z)Mn - (Eb /c 2 ) • Life of a star is based on a sequence of nuclear fusion reactions. • Heat produced counteracts gravitational attraction and prevents collapse. binding energy per nucleon Binding energy and mass loss A=total no. nucleons Z=total no. protons E b= binding energy Change from X to Y emits energy since Y is more tightly bound per nucleon than X. Fusion Fission X Y Fe Y X A Stellar Evolution and Supernovae • Series of collapses and fusions H => He => C => Ne => O => Si • Outer parts of star expand to form opaque and relatively cool envelope (red giant phase). • Eventually, Si => Fe: most strongly bound of all nuclei • Further fusion would absorb energy so an inert Fe core formed • Fuel in core exhausted hence star collapses. Stellar Evolution Schematic Complete Star a Red Supergiant 103 R core Nuclear Fusion Regions near Inert Fe Core Stellar Mass Ranges for Supernovae • Three possibilities: 2.0 < M star < 8 M 1.4 < M core < 1.9 M 8.0 < M star < 15 M Mcore > 1.9 M 15 M < Mstar Type I SN Type II SN Type II SN • If the star has < 2 M or the core is < 1.4 M, it undergoes a quiet collapse, shrinking to a stable White Dwarf. Stellar Mass Ranges (Cont.) Type I: Small cores so C-burning phase occurs catastrophically in a C-flash explosion and star is disrupted 2.0 < M < 8 M → Disintegration/no Neutron star Star Type II: More massive, so when Si-burning begins, star shrinks very rapidly 8 < M star < 15 M → Neutron Star 15 M < Mstar → Black Hole Stellar Collapse and Supernova Summary • • • • • Stars with a defined mass range evolve to produce cores that can collapse to form Neutron Stars Following nuclear fuel exhaustion, core collapses gravitationally; this final collapse supplies the supernova energy Collapse to nuclear density, in ≈ few seconds, is followed by a rebound in which the outer parts of the star are blown away The visible/X-ray supernova results due to radiation from this exploded material and later from shock-heated interstellar material Core may i. ii. iii. Disintegrate Collapse to a Neutron star Collapse to a Black Hole according to its mass which in turn depends on the mass of the original evolved star Energy Release in Supernovae • Outer parts of star require >10 44J to form a Supernova… how does the implosion lead to an explosion? • Once the core density has reached 1017 - 1018 kg m-3, further collapse impeded by nucleons resistance to compression • Shock waves form, collapse => explosion, sphere of nuclear matter bounces back. Shock Waves in Supernovae • Discontinuity in velocity and density in a flow of matter. • Unlike a sound wave, it causes a permanent change in the medium • Shock speed >> sound speed - between 30,000 and 50,000 km/s. • Shock wave may be stalled if energy goes into breaking-up nuclei into nucleons. • This consumes a lot of energy, even though the pressure (nkT) increases because n is larger. Importance of Neutrinos • Neutrinos carry energy out of the star • They can - Provide momentum through collisions to throw off material. - Heat the stellar material so that it expands. • Neutrinos have no mass (like photons) and can traverse large depths without being absorbed but they do interact at typical stellar core densities r > 1015 kg m-3 Neutrinos (Cont.) • Thus a stalled shock wave is revived by neutrino heating. • Boundary at ~150 km: – inside → matter falls into core – outside → matter is expelled. • After expulsion of outer layers, core forms either: – neutron star (Mcore < 2.5 M) or – black hole (depends on gravitational field which causes further compression). • Neutrino detectors set up in mines and tunnels must be screened from cosmic rays. Neutrinos (Cont.) • Neutrinos detected consistent with number expected from supernova in LMC in Feb 1987. • Probably type II SN because originator was massive B star (20 M) • Neutrinos are rarely absorbed so energy changed little over many x 10 9 years (except for loss due to expansion of Universe)… thus they are very difficult to detect. • However density of collapsing SN core is so high however that it impedes even neutrinos!!! Supernovae 45 • Energy release ≤ 10 J in type I and II SN • Accounts for v >10,000 km/s initial velocity of expanding Supernova Remnant (SNR) shell. • Optically the “star” brightens by more than 10 mag in a few hours, then decays in weeks months Explosive nucleosynthesis: • Reactions of heavy nuclei produce ~1 M of 56 56 56 Ni which decays to Co and Fe in ~ months. • Rate of energy release consistent with optical light curves (exponential decay; t ~ 50 - 100 d) Shock Expansion • At time t=0, mass m 0 of gas is ejected with velocity v0 and total energy E 0. • This interacts with surrounding interstellar material with density r0 and low temperature. Shock front, ahead of ‘heated’ material R Shell velocity much higher than sound speed in ISM, so shock front of radius R forms. ISM, r 0 • System radiates (dE/dt) rad. Note E0 ~10 41-45 J Supernova Remnants Development of SNR is characterized in phases – values are averages for “end of phase” Phase I II III Mass swept up (M) 0.2 180 3600 Velocity (km/s) 3000 200 10 Radius (pc) 0.9 11 30 Time (yrs) 90 22,000 100,000 Phase IV represents disappearance of remnant SNR Development - Phase I • Shell of swept-up material in front of shock does not represent a significant increase in mass of the system. • ISM mass within sphere radius R is still small. 4 3 m0 r 0 R (t ) 3 (1) • Since momentum is conserved: 4 3 m0 v0 (m0 r 0 R (t )).v(t ) 3 (2) • Applying condition (1) to expression (2) shows that the velocity of the shock front remains constant, thus : v(t) ~ v 0 R(t) ~ v 0 t Supernova 1987A • B3 I Star exploded in February 1987 in Large Magellanic Cloud (LMC). • Shock wave now ~ 0.13 parsec away from the star, and is moving at vo~ 3,000 km/s. SNR Dusty gas rings light up •Two sets of dusty gas rings surround the star in SN1987A, thrown off by the massive progenitor. •These rings were invisible before – light from the supernova explosion has lit them up. Shock hits inner ring The shock has hit the inner ring at 20,000 km/s, lighting up a knot in the ring which is 160 billion km wide. Chandra X-ray Images of SN 1987A • X-ray intensities (0.5 – 8.0 keV) in colour with HST Ha images as contours • Low energy X-rays are well correlated with optical knots in ring – dense gas ejected by progenitor? • Higher energy X-rays well correlated with radio emission – fast shock hitting circumstellar H II region? • No evidence yet for emission from central pulsar Phase II - adiabatic expansion Radiative losses are unimportant in this phase - no exchange of heat with surroundings. Large amount of ISM swept-up: 4 3 m0 r 0 R (t ) 3 (3) Thus (2) becomes : 4 3 m0 v0 r 0 R (t )v(t ) since mo is small 3 4 dR(t ) (4) 3 r 0 R (t ) 3 dt Integrating: 4 (5) m0 v0t r 0 R (t ) 3 Substituting (4) for movo in (5) R(t) = 4v(t).t v(t) = R(t)/4t • Taking a full calculation for the adiabatic shock wave into account for a gas with g = 5/3: 1 5 2 5 R(t ) E0 R(t ) 1.17 t and v(t ) 0.4 t r0 • Temperature behind the shock, T v2, remains high – little cooling 3 m 2 T v 16 k • Typical feature of phase II – integrated energy lost since outburst is still small: dE dt E 0 dt RAD N132D in the LMC • SNR age ~ 3000 years • Ejecta from the SN slam into the ISM at more than 2,000 km/s creating shock fronts. • Dense ISM clouds are heated by the SNR shock and glow red. Stellar debris glows blue/green SNR N 132D XMM CCD Image and Spectrum • X-ray image gives a more coherent view of the SNR • Lower ion stages (N VII, C VI) show T ~ 5 MK gas in ISM filaments at limb • Higher ion stages (Fe XXV) show T ~ 40 – 50 MK gas more generally distributed Phase III - Rapid Cooling • SNR cooled, => no high pressure to drive it forward. • Shock front is coasting 4 3 R r 0 v = constant 3 • Most material swept-up into dense, cool shell. • Residual hot gas in interior emits weak Xrays. XMM X-ray Observations: SNR DEM L71 • Remnant in Large Magellanic Cloud (LMC): 0.7 – 1.0 keV d = 52 pc; diam → 10 pc; age → 104 yr • Just entering Phase III: vshock ~ 500 km/s; Tinterior ~ 15 MK, Tshell ~ 5 MK • Shell emission dominates (XMM CCD spectra) • Emission line spectrum from XMM RGS shows: - thermal nature of the plasma Chandra X-ray image: shell & centre - element abundances characteristic of LMC Shell Interior XMM Reflection Grating Spectrometer (RGS) spectrum XMM CCD Spectra Phase IV - Disappearance • ISM has random velocities ~10 km/s. • When velocity(SNR) is ~ 10 km/s, it merges with ISM and is ‘lost’. • Oversimplification!!! - magnetic field (inhomogeneities in ISM) - pressure of cosmic rays Example – Nature of Cygnus Loop - passed the end of phase II - radiating significant fraction of its energy Rnow ~ 20pc vnow ~ 115 km/s (from Ha) 16 lifetime, Rnow 20 3 10 0.4 t ~ 0.4 sec 5 vnow = 2 x 10 12 seconds 1.15 10 = 65,000 years 3 Assuming v0 = 7 x 10 km/s and r0 = 2 x 10 -21 kg m-3 , from (5) we find that m0 ~10 M Density behind shock, r, can reach 4r 0 , (r0 is ISM density in front of shock. 3 m 2 Matter entering shock heated to: T v 16 k ( m = av. mass of particles in gas) For fully ionized plasma (65% H; 35% He) 5 T 1.45 10 v 2 (6) Cygnus Loop: vnow ~ 105 m/s => T ~ 2 x 10 5 K (from (6)) But X-ray observations indicate T ~ 5 x 10 6K implying a velocity of 600 km/s. Thus Ha filaments more dense and slower than rest of SNR. Young SNRs • Marked similarities in younger SNRs. • Evidence for two-temp thermal plasma - low-T < 5 keV (typically 0.5-0.6 keV) - high-T > 5 keV (T = 1.45 x 10 -5v 2 K) • Low-T - material cooling behind shock High-T - bremsstrahlung from interior hot gas Older SNRs • A number of older SNRs (10,000 years or more) are also X-ray sources. • Much larger in diameter (20 pc or more) • X-ray emission has lower temperature essentially all emission below 2keV. • Examples : Puppis A, Vela, Cygnus Loop all Crab-type SNRs. Crab Nebula • 1st visible/radio object identified with cosmic X-ray source. • 1964 - lunar occultation => identification and extension • Well-studied and calibration source (has a well known and constant power-law spectrum) Crab Nebula Exploded 900 years ago. Nebula is 10 light years across. • No evidence of thermal component • Rotational energy of neutron star provides energy source for SNR (rotational energy => radiation) • Pulsar controls emission of nebula via release of electrons • Electrons interact with magnetic field to produce synchrotron radiation Spectrum of the Crab Nebula Watts per sq m per Hz Log flux density Radio -22 IR-optical X-ray Log n (Hz) -32 8 10 16 also g-rays detected up to 20 2.5x1011 eV • Summarizing: Bnebula ~ 10-8 Tesla to produce X-rays nm ~ 1018 Hz (ie. peak occurs in X-rays) E e- ~ 3 x 1013 eV tsyn ~ 30 years • Also, expect a break at frequency corresponding to emission of electrons with lifetime = lifetime of nebula. Should be at ~10 15 Hz (l~3000Angstroms). This and 30 year lifetime suggest continuous injection of electrons. SUPERNOVAE END OF TOPIC